1. Field of Invention
This invention relates generally to an electronic timepiece and radiation monitor assembly which indicates time and also the intensity of radiation to which the user is exposed, and in particular to a wrist watch and radiation monitor assembly in which the timekeeping components cooperate symbiotically with radiation monitoring components whereby the watch not only displays time and other time related data, but also indicates the existing intensity of penetrating radiation to which the wearer is exposed and the accumulated dosage thereof.
2. Status of Prior Art
The keeping of time by means of electronic timepieces, and the monitoring of radiation by means of geiger counters or other radiation detectors both involve pulse counting procedures. A timekeeper and a radiation monitor together afford data vital to a person living in a period in which virtually all activity is time scheduled and is carried out in an environment that may contain dangerous levels of radioactivity.
In the present invention, the functions of timekeeping and radiation monitoring are integrated into a single portable instrument which may be wrist-borne or otherwise carried on the person. Hence for background purposes, we shall first separately consider these functions, and then explain why it is desirable to integrate them into a single, highly compact instrument.
When exposed to X-rays, gamma rays or other forms of penetrating radiation such as radiation fallout following a nuclear test or accident, living organisms then become subject to the damaging effects of such radiation. These effects are collectively referred to as radiation hazards. The concern with radiation hazards is growing and is now international in scope, and it continues to intensify because of recently highly-publicized incidents.
Thus, the catastrophe at Chernobyl resulting from a malfunction at a nuclear power plant produced radioactive fallout whose boundaries extended from beyond those of the USSR. And the Three Mile Island disaster in Pennsylvania was by no means limited in its consequences to this state. Nor is the presence of hazardous radon gas limited to any one geographical region.
The present recommendation of the United States National Committee on Radiation Protection is that the permissible gamma dose should not exceed 300 milliroentgens per week with a maximum daily exposure of 100 milliroentgens. Assuming a 40 hours work week for personnel in the radiation industry, 300 MR/week equals 7.5 MR/hr. An ability to detect radiation at about one-tenth this rate is normally considered necessary for personnel monitoring. One roentgen/hour is approximately equivalent to one rad/hour.
Because gamma rays deposit energy in an absorbing medium in a way which is dependent upon their energy, a given radiation dosage is effected with different amounts of gamma rays of one energy compared to another energy. For example, it requires approximately 5(10).sup.4 gammas/sec/cm.sup.2 (at 0.1 MEV) to produce 60 milliroentgens in 8 hours (7.5 MR/hr) compared to 4(10).sup.3 gammas/sec/cm.sup.2 at 1 MEV. The 1 MEV gammas deposit approximately 10 times the energy in an absorbing medium as do the 0.1 MEV gammas. Because gamma rays are absorbed exponentially in material--that is I=I.sub.o (1-e.sup.-.mu.x)--for a given incident flux of I.sub.o and an absorption factor .mu., the resultant absorbed intensity I can be calculated for any material thickness X. The absorption coefficient fo 0.1 MEV gammas is approximately 0.1/cm and therefore for a 2.8(10).sup.-3 cm thick solid state detector, an incident intensity of 7000 gammas/sec/cm.sup.2 (1 MR/hr) and a 1 cm.sup.2 detector, the number of absorbed gammas is 1.75/sec.
Because high speed (high count rate) amplifiers require more power than low speed designs, an upper counting rate limit of 10.sup.4 counts/sec is dictated by our low power watch battery constraint. This rate would be equivalent to approximately 5 r/hour however, and therefore gamma and x-ray detection is possible over a wide dynamic range from 1 MR/hour to 5 r/hour.
The human sensory organs are highly responsive to light, sound and other stimuli, but are generally insensitive to penetrating radiation. It is for this reason that one may receive a lethal dose of radiation without in any way feeling it. Various forms of cancer are induced by excessive exposure to radiation without the individual experiencing any sensation. The biological consequences of radiation take either a genetic or a somatic form. Only low levels of radiation are needed to effect genetic mutations or alterations in heredity, an these only show up in future generations. With somatic injury, the effects of radiation is on the individual's body cells.
The increasing use of X-rays in medical and dental diagnosis, the rising amount of waste products from nuclear power facilities, and the widespread industrial applications for radioactivity make it more necessary than every to monitor the the radioactivity to which an individual is subjected. There is scarcely any environment today which is altogether free of penetrating radiation. Thus, typical homeowners have reason to be concerned about radiation in the vicinity of their TV sets or microwave ovens, and possibly radon gas emanating from the ground on which their houses are anchored. And, of course, if a home is situated in the vicinity of a nuclear power plant, there is greater cause for anxiety.
Various standards or radiation units are now used to measure the intensity of radiation emitted by radioactive nuclei. The "curie" measures the rate at which radioactive material emits radiation. A sample is said to have an activity of one curie if 3.7(10).sup.10 of its nuclei disintegrate in one second.
The "roentgen" is a measure of the energy deposited in a region through which radiation has passed. When electrically-charged particles produced by radioactive nuclei pass through a medium, they knock electrons out of the atoms in their path to create ions, and the number of ions so produced is proportional to the total energy deposited. The dosage is said to be one roentgen if 2.08(10).sup.9 ions are produced in one CM.sup.3 of air. A typical dental X-ray examination of the jaw delivers 5 roentgens. Since ionization within biological cells can kill these cells, there are now strict safety standards set for X-ray dosages.
Other radiation dosage units are predicated on the energy absorption corresponding to irradiating body tissue by one roentgen of X-radiation. Thus, the roentgen equivalent physical unit, abbreviated as REP, corresponds to the energy absorption of 93 ergs/gram by tissue through which ionizing radiation passes. The REP unit has been replaced by the RAD unit, and this corresponds to energy absorption of 100 ergs/gram of body tissue.
Relative biological effectiveness (RBE) is a weighting factor that is equal to unity for X-rays. RBE expresses the degree to which a given amount of radiation is more or less effective in producing a biological effect than X-rays of the same RAD. The "roentgen equivalent mammal" (REM) unit, defined originally in terms of the REP, is the amount of any given radiation producing the same effect as one REP of X-rays. The current definition of REM is one REM equals [1/RBE]RAD.
Radiation monitors which make use of geiger counters and other radiation detectors are well known. Thus, the Snaper et al. U.S. Pat. No. 4,482,442 senses ionizing radiation by means of a geiger counter whose pulses are applied to a processor to provide radiation intensity and dosage readings. When a charged particle traverses a geiger counter, an electrical pulse is produced, the count rate being an index to the intensity of radiation.
The Waechter et al. U.S. Pat. No. 4,536,841 discloses a battery-operated portable dosimeter using a microprocessor coupled to a neutron detector to calculate and display the accumulated dosage. A similar arrangement is disclosed in the Mastain et al. U.S. Pat. No. 4,480,311. Also of background interest is the Waechter et al. U.S. Pat. No. 4,550,381.
The term "solid state watch," as used herein, is limited to timepieces having an electro-optic time display and no moving parts. In a typical solid state watch such as that disclosed in the Sagarino U.S. Pat. No. 4,033,110, low-frequency electrical pulses derived from a high-frequency piezoelectric crystal-controlled time base serve to actuate a multi-station display formed by light-emitting diodes (LED) or by liquid-crystal display elements (LCD).
In such solid state watches, the output of the crystal-controlled oscillator is fed to a frequency converter formed by a chain of frequency-divider stages. The low-frequency timing pulses yielded by the converter are applied to a miniature microprocessor or time computer that counts the input train of pulses, encodes it in binary form and then decodes and processes the resultant data to provide appropriate activating signals for the LCD or LED display stations.
In the modern multi-mode solid state wrist watch, the arrangement is such as to provide calendar as well as time readings. Thus, the watch in one mode will display the existing time, say, 12:30 AM, and in another mode, the month and date, say, 05:23, meaning the 23rd day of May.
A multi-mode solid state watch of this type produces one pulse per second to provide a minute indication when sixty second pulses are counted, and to provide an hour indication when 3600 second pulses are counted. It produces a change in date when 3600.times.24 pulses are counted, and a change in month when 3600.times.24.times.28 pulses, or 3600.times.24.times.30 pulses, or 3600.times.24.times.31 pulses are counted.
Where an analog display is required, the electronic watch also includes a time base and a frequency divider which yields one pulse per second, but these pulses are applied to a stepping motor which drives the gear train that turns the analog hands of the watch. In some cases, a modern electronic watch will include both digital and analog displays, the former being used to provide calendar indications.
Almost everybody today wears a wrist watch so that he can keep tabs on the time and schedule his activity accordingly. But while many of these same persons are cognizant of the hazards of radiation, unless they are working in an environment known to present significant radiation problems, such as a medical diagnostic laboratory, they do not also have on their person a radiation monitor.
One practical reason for the general absence of radiation monitors in a world concerned with radiation hazards is that even those battery-powered monitors which are classified as portable are relatively bulky and conspicuous. Thus, while in a medical X-ray laboratory, one would expect to have a radiation monitor available, a person visiting an office, a home or any industrial plant would find it embarrassing to be seen carrying along a radiation monitor. Moreover, commercially available radiation monitors are not waterproof and cannot be immersed in a liquid to determine whether the liquid is radioactive, even though some water supplies have been found to exhibit radioactivity.
Yet, as previously pointed out, because of the prevalence of penetrating radiation from various man-made and natural sources, individuals today have reason to be concerned not only with the existing intensity of radiation to which they are exposed, but they have greater reason to be worried about long-term cumulative effects. While in the course of any day an individual may only receive a small dose of penetrating radiation (even our Sun periodically emits bursts of penetrating radiation), over a period of, say, one to three years, the cumulative dosage may be considerable and approach a dangerous total. The typical portable radiation detector does not give long term cumulative dosage values.
Also, where high levels of radiation are detected, for example, because of leakage in an industrial plant, it is important for individuals who carry the detector to know how much time they have to escape from this environment in order to avoid an excessive dosage. Such information is not furnished by the ordinary monitor.